A gal at the quilt shop told me to cut a small piece off, light it with a match. If it is a soft ash, it is cotton. I haven't done it, but she claims if it is not 100% cotton the ash is harder/dense. Maybe someone else can answer it better

yes the ash test is the fastest. if it has any sort of poly or rayon it will melt and leave a blackish glob, how ever large or small you test piece is. Please dont touch it till proper time has elapsed for cooling.